1. Field of the Invention
The present invention relates to an open check circuit for sensor applications. The open check circuit can be implemented in a sensor-adapted thermometer, a hygrometer or a heating apparatus for use in home and industry, as well as home appliances such as electric cookers, electric refrigerators, air conditioners, and humidifiers in which sensors are commonly used.
2. Description of the Prior Art
In general, the conventional sensing circuit is comprised of a fixed circuit, i.e., voltage divider circuit having a resistor and a sensor, or a voltage divider and an operational amplifier. In this circuit, as the variation range of the sensing grows larger, the performance of the open check becomes more difficult.
In Japanese patent Laid-open Gazette No. SHO 60-18006 is disclosed a temperature-sensing circuit which has wide hysteresis characteristics in which a thermal sensor such as thermistor is used. When the sensor is set to a level above the predetermined value of the resistor, the input value of the controller will be saturated. In this case, it cannot determine whether the operation of the sensing circuit is in a normal state, or an abnormal state where the sensor is defective.
Furthermore, when an open check for a sensor cannot be effected, the system will continue to operate even in the absence of the sensor. Thus, fatal damage to the system results due to the malfunction of a sensing circuit. Consequently, the open check for a sensing circuit should always be required.